The present invention is based on the principle of utilizing a fibreoptical Bragg grating. A Bragg grating is single modus fibre with permanent periodic variation of the refractive index over a fibre length of, for example 0.1 to 10 cm. The variation in the refractive index is established by illuminating the fibre with a UV laser. A Bragg grating reflects light with a wavelength that depends upon the refractive index and the space related period of the variation of the refractive index (the grating period), while light beyond this wavelength will pass through the grating more or less unhindered. The light reflected by the Bragg grating will exhibit a wavelength that varies as a function of a measurable quantity that changes the refractive index of the fibre material grating and/or the fibre length in the gratina zone (grating period). Tension in the fibre or temperature variations will therefore lead to a change of the wavelength of the light reflected by the Bragg grating.
For practical purposes one can, for example measure the temperature in the region -100.degree. C to +250.degree. C with (in the order of) 20 different points along the fibre for fibres with a length of up to 50-100 km. Using various multiplexing techniques, the number of measurement points can be increased. Examples of areas of application are temperature surveillance of power cables, pipelines, electrical transformers, engines and temperature monitoring of industrial processes.
A number of devices for measurement of tension in mechanical constructions exist. For special purposes where there is little space available, high temperature. high tension and so forth, all known devices for measurement of tensile forces have functional disadvantages. For example present measurement of tension under water is made with tensile sensitive sensors based on electrical elements, which in such environments exhibit low reliability. For other areas of application there may be little space available for installing extra components, such as tension sensors based on electrical induction or capacity (typical diameter 10-20 mm). Another example is the surveillance of darn with sensors based on electrical strain gauges. In such connections lightening strikes have sometimes rendered the sensor elements or the electronic circuits passive, and thus disabled the tension surveillance.
Accordingly there is a need for a tension sensor with mainly passive components that can be utilized in difficult environments and narrow spaces.
The objective of the present invention is to provide a device of this type for tension measurement in and on mechanical constructions.